As is known in the art, sensors responsive of electromagnetic energy in both the optical and infrared spectrums are used in a wide variety of applications. One such application the sensors are arranged in an array, as for example as a focal plane array (FPA), to provide imagining. Outputs of these sensors are typically fed to read out electronics (ROE, sometimes also referred to as read out integrated circuit (ROIC) for processing. Typically the ROE is fabricated as an integrated circuit mounted behind the sensors.
In some applications, the array may be exposed to high intensity electromagnetic radiation, such as radio frequency radiation that may interfere with the ROE; referred to as Electromagnetic Interference or EMI. Therefore, in some applications, an electromagnetic interference protection structure is required to allow the infrared and/or optical energy to impinge upon the sensors while preventing other radiation from impinging upon the ROE. One such structure uses a thin film of optically transparent, highly conductive Indium Tin Oxide or Indium Oxide, which is thermally evaporated coating on the surface of the array of sensors. The Indium Tin Oxide, being electrically conductive, forms an electromagnetic shield over the ROIC or the FPA and prevents RF energy from reaching the ROIC. However, the Indium Tin Oxide coating does not provide adequate transparency in infrared part of the spectrum due to IR absorption of free carriers present in the material (see R. A. Synowicki, “spectroscopic ellipsometery characterization of Indium tin oxide film microstructure and optical constants” Thin Solid Films, 313-314, 1998, pp. 394-397, J. R. Bellingham, W. A. Phillips and C. J. Adkins, “Amorphous indium oxide” Thins Solid Films 195, 1991 pp. 23-31; and Tze-chiang Chen, Tso-ping Ma, and Richard C. Barker “infrared transparent and electrically conductive thin film of In2O3” Applied Physics Letters, Vol. 43, No. 10, Nov. 15, 1983, pp. 901-903) and the photoabsorption non-linearly increases in infrared (see P. Blake, P. D. Brimicombe, R. R. Nair, T. J. Booth, D. Jiang, F. Schedin, L. A. Ponomarenko, S. V. Morozov, H. F. Gleeson, E. W. Hill, A. K. Geim, K. S. Novoselov (2008). “Graphene-Based Liquid Crystal Device”. Nano Letters 8 (6): 1704-1708). FIG. 1 shows the calculated extinction coefficient in the visible and near IR for Indium oxide film calculated from ellipsometric data, clearly showing the strong absorption in near infrared and a sharp increase in absorptivity in the infrared spectrum.
The inventor has recognized that carbon based two-dimensional (2D) crystalline graphene is composed of one atomic layer material and as such displays remarkable electronics and optical properties which can be exploited in numerous applications, because such a film of graphene is one atomic layer, is highly electrically conductive and does not have a significant absorption in optical and infrared spectrum and hence can be used as a protection layer for optical and IR devices. Further, because as noted above, ROE in many optical and infrared array detectors suffer from interference from stray radio frequency radiation and need to be protected to reduce noise and/or prevent damage to the electronics and hence graphene can provide such protection. Due to its special bandgap properties, graphene has zero bandgap energy and as such can absorb all optical photons with energies above zero, i.e. very long wavelength infrared, all the way up to optical and UV photons with energies in excess of several eV. However, owing to its one layer thickness, the total absorption remains very low. It has been reported in the literature [see K. Grodecki, A. Drabinska, R. Bozek, A. Wysmolek, K. P. Korona, W. Strupinski, J. Borysiuk, R. Stepniewski and J. M. Baranowski, “Optical Absorption and Raman Scattering Studies of Few-Layer Epitaxial Graphene Grown on 4H—SiC Substrates” Proceedings of the XXXVIII International School and Conference on the Physics of Semiconductors “Jaszowiec” 2009] that monoatomic layer graphene films have absorption of ˜2.3% (see FIG. 2) in the visible and near infrared. However, since the film is single or perhaps a few atomic layers, the total absorption remains very low (at less than 10%) and thus the film is highly transparent in the optical spectrum.
I have calculated the absorption and transmission in RF region of the spectrum for a thin film based on a transmission line model and FIG. 3, shows this data as a function of film sheet resistivity. For EMI protection the film needs to be able transmit less than 10% of the impinging RF radiation, while dissipating and reflecting greater than 90% of it. This restriction places a further requirement on sheet resistivity of less than 100 Ohm/square on the EMI protection film. Further, we have calculated the EMI shielding effectiveness as: Shielding Effectiveness=10 Log(Incident power/Transmitted power) and plotted it as function of film sheet resistivity in FIG. 4, clearly confirming the sheet resistivity requirement for the EMI film at less than 100 Ohm/square. In FIG. 4, Pi/Pt is the ratio of radio frequency (RF) power incident on the thin film of graphene (Pi) to the radio frequency (RF) power transmitted through the thin film of graphene (Pt).
In accordance with the present disclosure, a detector structure is provided, comprising: a sensor for detecting energy impinging on the structure in the infrared and/or optical frequency band; an electronics section disposed behind the sensor for processing electrical signal produced by the sensor in response to the sensor detecting the infrared and/or optical energy; and an electrically conductive layer for inhibiting electromagnetic energy outside of the visible and infrared portions of the spectrum, such electrically conductive layer being disposed between the impinging energy and the electronics section, such layer having a transmissivity greater than 90 percent in the visible and infrared portions of the spectrum and being reflective and dissipative to portions of the impinging energy outside of the visible and infrared portions of the spectrum.
In one embodiment the layer is graphene.
In one embodiment, a detector structure is provided, comprising: a sensor for detecting energy impinging on the structure in the infrared and/or optical frequency band; an electronics section disposed behind the sensor for processing electrical signal produced by the sensor in response to the sensor detecting the infrared and/or optical energy; and an electrically conductive layer having a substantially constant absorptivity to electromagnetic energy within the visible and infrared portions of the spectrum with less than 5% variation in absorptivity in the visible and infrared spectrum.
In one embodiment, a detector structure is provided, comprising: a sensor for detecting energy impinging on the structure in the infrared and/or optical frequency band; an electronics section disposed behind the sensor for processing electrical signal produced by the sensor in response to the sensor detecting the infrared and/or optical energy; and a layer of graphene.
In one embodiment, the layer is less than four atomic layers of graphene.
The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.